1. Field of the Invention
The present invention relates to a toner reservoir with a wiper mechanism for wiping a sensitive surface of a toner sensor provided for the toner reservoir.
2. Description of the Related Art
There are used various electrostatic image generating apparatus, including electrostatic copiers, laser printers, laser facsimile machines and the like. A typical electrostatic image generating apparatus uses a photosensitized medium, in the form of a drum or a belt, having a photosensitized surface made of photoconductive insulating material. A charging unit is used to place a uniform electrostatic charge over the photosensitized surface preparatory to imaging. Then, a desired light image is either projected by an optical system or drawn by a laser beam scanner on the photosensitized surface, to form an electrostatic latent image on the surface. Thereafter, the latent image is developed with a developing material, powdery material referred to in the art as toner, to form a powder image on that surface. The powder image is then transferred to and fixed onto a support surface, such as a surface of a sheet of paper.
For developing the latent image, such electrostatic image generating apparatus include a developer unit, which is designed to apply toner to the photosensitized surface having a latent image formed thereon, and a toner dispenser for dispensing toner into the developer unit. A typical toner dispenser comprises a dispenser roller cooperating with a toner reservoir, which may be also referred to as a toner hopper. The dispenser roller may comprise a foam roller, for example. During development process, toner is either continuously or periodically dispensed from the toner reservoir into the developer unit by means of the dispenser roller.
The toner reservoir reserves a supply of toner therein. After use of the apparatus for a certain length of time, such as several weeks or months, the supply of toner in the toner reservoir is depleted, so that the toner level in the toner reservoir reduces to reach a predetermined minimum level, above which the toner level has to be kept in order to ensure quality of images produced by the apparatus.
Many toner reservoirs used in recent electrostatic image generating apparatus are provided with a toner level sensor for sensing the toner level in the toner reservoir, so as to produce a low-toner-level alarm when it detects the toner level in the toner reservoir below the minimum level. This alarm is typically a visual alarm which may be provided by an indicator lamp, a visual display or the like. The user is informed thereby of the necessity for toner replenishment in the toner reservoir.
A typical toner level sensor uses a toner sensor having a sensitive surface. The sensitive surface is disposed in the toner reservoir at the height of the minimum level. The toner sensor detects whether its sensitive surface is in contact with the amount of toner reserved in the toner reservoir. If so, the toner level in the toner reservoir is above the minimum level. Otherwise, it is below the minimum level so that toner replenishment is required.
Because the toner sensor environment within the toner reservoir is dirty and severe due to existence of contaminous toner, an oscillatory sensor comprising a piezoelectric crystal element, which is highly immune to such environment, is preferably used together with associated electronic circuitry for supplying radio-frequency (RF) voltage to and measuring impedance of the piezoelectric crystal element. The piezoelectric crystal element has surfaces for oscillation to be induced by the applied RF voltage, one of which is exposed to serve as the sensitive surface of the toner sensor. Typically, the frequency of the RF voltage is selected to the natural resonance frequency of the piezoelectric crystal element. When the toner level in the toner reservoir is above the minimum level, the sensitive surface of the sensor is submerged in and thus in contact with the amount of toner reserved in the toner reservoir, which results in a shift of the resonance frequency of the piezoelectric crystal element from its natural resonance frequency. This in turn leads to a variation in its impedance, which is detected and used for determination whether the toner level is above the minimum level.
There arise a problem, however, from the inherent, cohesive nature of toner. After use of a toner reservoir for a certain long time, such as several months, there is often found a sticky layer r of cohered toner formed on the sensitive surface of the toner sensor. Further, the cohesion of toner can be promoted by the oscillatory pressure imposed by the oscillating sensitive surface of the toner sensor. Usually, the thickness of cohered toner layer formed on the sensitive surface of the sensor is small, but enough to shift the resonance frequency of the sensor to cause a substantial impedance variation, which may often result in a failure to detect a toner level below the minimum level.
One solution for this problem is a manual cleaning operation by the operator to wipe and clean the sensitive surface of the sensor; however, this is a highly dirty operation possibly creating a cloud of toner around the toner reservoir which is open in order to clean the sensitive surface of the sensor, and such a cloud of toner will contaminate parts of the machine using the toner reservoir as well as hands and clothes of the operator. The use of a motor-driven cleaning mechanism is another solution; however, such a mechanism is bulky and costly.